Advances are being made in application of a current excitation type light-emitting element in which an organic compound is used as a light-emitting substance, i.e., an organic EL element, to light sources, lighting, displays, and the like.
As is known, in an organic EL element, the generation ratio of excitons in a singlet excited state to excitons in a triplet excited state is 1:3. Thus, the limit value of internal quantum efficiency of fluorescence, which is emitted by conversion of a singlet excited state into light emission, is 25%, while phosphorescence, which is emitted by conversion of a triplet excited state into light emission, can have an internal quantum efficiency of 100% when energy transfer via intersystem crossing from a singlet excited state is taken into account. In view of the above, an organic EL element (also referred to as a phosphorescent light-emitting element) in which a phosphorescent material is used as a light-emitting substance is selected in many cases so that light is emitted efficiently.
Most of the substances capable of efficiently converting a triplet excited state into light emission are organometallic complexes, and in most cases, central metals of the organometallic complexes are rare metals whose production is small. The price of rare metals is high and greatly fluctuates, and supply thereof might be unstable depending on the global situation. For this reason, there are some concerns about cost and supply regarding phosphorescent light-emitting elements.
To cause conversion of a triplet excited state into light emission, delayed fluorescence can also be utilized. In this case, not phosphorescence but fluorescence is obtained because reverse intersystem crossing from a triplet excited state to a singlet excited state is utilized and the light emission occurs from a singlet excited state. This is readily caused when an energy difference between a singlet excited state and a triplet excited state is small. Emission efficiency exceeding the theoretical limit of emission efficiency of fluorescence has been actually reported.
It has also been reported that an exciplex (excited complex) formed by two kinds of substances was utilized to achieve a state where an energy difference between a singlet excited state and a triplet excited state is small, whereby a high-efficiency light-emitting element was provided.